The invention relates to the field of lubricants and has more particularly as subject-matter that of aqueous lubricants of use in the working or shaping of metals.
Operations in which metals are worked or shaped require the use of a lubricant in order to reduce the stresses between the component to be worked and the tool, to remove the turnings and the debris, to cool and to control the surface condition of the component or of the metal sheet which is being worked. Oil-based lubricants have conventionally been used. They are whole oils or emulsions to which have optionally been added lubricity agents, antiwear (AW) additives and/or extreme pressure (EP) additives. The lubricity agents form a monomolecular covering on the surface to be lubricated and thus reduce wear and friction. When the frictional conditions become more severe, the rise in the temperature results in a desorption of these lubricity agents, and AW or EP additives (generally compounds comprising phosphorus, chlorine or sulphur) are then necessary to lubricate the contact. Under high loads, AW additives make it possible to greatly reduce the wear of the components in contact; EP additives can generate a degree of wear but they prevent the phenomena of welding and of adhesion. Whole oils have excellent lubricating properties but, when throughputs are high, the removal of heat requires the use of emulsions. However, the use of emulsions is also tending to be reduced as, over time, they decompose and produce unpleasant smells.
For this reason, there is increasing use of aqueous fluids, which are either synthetic fluids (aqueous solutions based on water-soluble additives) or semisynthetic fluids (oil-in-water microemulsions comprising a large amount of emulsifiers). However, while aqueous fluids are very efficient in removing heat and have an improved resistance to bacterial proliferation, their use is often restricted to metal working operations where the frictional and wear conditions are not excessively severe. This is because, as AW and EP additives have been developed for oils, very few of them are soluble in water and suitable for aqueous fluids.
There is an overabundance of oil-soluble AW or EP additives but the number of water-soluble AW or EP additives is markedly more restricted. Nevertheless, it is possible to distinguish two main categories of water-soluble additives: phosphorus compounds and sulphur compounds.
The most widely used among phosphorus compounds are phosphoric acid esters obtained by reaction of an ethoxylated or nonethoxylated fatty alcohol with P2O5. They are soluble in water by neutralization in the form of an alkali metal, ammonium or alkanolamine salt or alternatively by virtue of their ethoxylated part. These esters, which are widely employed in aqueous fluids for the working or shaping of metals because of their AW, emulsifying and corrosion inhibiting properties, are not, however, suitable for operations where the frictional and wear conditions are very severe.
Mention may be made, among sulphur compounds, of alkylthio acids, which are monosulphides composed of an alkyl chain, generally comprising between 6 and 18 carbon atoms, and of a carboxylic acid group. These compounds, which are soluble in water by neutralization in the form of alkali metal, ammonium or alkanolamine salts, are used in aqueous lubricants for the working or shaping of metals for their AW and corrosion inhibiting properties. However, their EP capability is insufficient for very severe operations.
R. W. Mould et al., in Lubr. Eng., 1977, 33(6), 291-298, have reported the EP properties of a few water-soluble sulphur additives, such as sodium salts of thiosalicylic, 2-mercaptopropionic, 2,2xe2x80x2-dithiodibenzoic, 2,2xe2x80x2-dithiodipropionic and dithiodiglycolic acids. The use of salts of 3,3xe2x80x2-dithiodipropionic acid or of dithiodiglycol as EP additives for aqueous lubricants has formed the subject-matter of Patents EP 288 375, JP 63-265997 and EP 183 050, it being possible for 3,3xe2x80x2-dithiodipropionic acid to be used in combination with alkylthio acids for aqueous chasing lubricants (JP 10-110181). Very recently, synergies with regard to the EP capability between acids of this type and orthophosphoric acid have been observed (WO 9808919).